Since the 1800's fingerprint information has been collected from human fingers and hands by applying ink to the fingers and/or hands, and then pressing or rolling on paper or card stock. For purposes of this document, the term fingerprint means the skin surface friction ridge detail of a single fingerprint, partial fingerprint or any portion of the skin surface friction ridge of up to and including the entire hand or foot.
In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and acoustic (including ultrasound) methods. Methods based upon capacitance and acoustics have proven to be the most accurate, because they are virtually immune to the effects of grease, dirt, paint, ink and other contaminants. Capacitance sensors may offer an advantage over acoustic methods in that they may be able to offer improved imaging in cases where there is poor acoustic impedance matching between the friction skin of the fingerprint and the scanner's platen, such as may be encountered when the friction skin is very dry.
Ultrasonic scanning systems often employ a piezoelectric transducer that sends longitudinal wave energy through transmitting media. At each media interface some of the energy reflects back toward the transducer. The reflected energy is received at the transducer and may be used to measure the exact distance traveled by the pulse going and returning for each reflecting material interface. As such, it is necessary to distinguish energy reflected by the object to be imaged from energy that has been reflected by other objects. To do so, a process called range gating (biasing) may be used. The returning signal that has been identified via range gating for further processing may be converted to a digital value representing the signal strength. Graphically displaying this information creates a contour map of the object (human finger or skin surface) that is in contact with the platen surface, and the depth of any gap structure (fingerprint valleys) detail may be displayed as a gray-scale bitmap image.
The capacitance method relies on the fact that capacitance is a function of the distance between capacitance plates, i.e., the TFT (Thin Film Transistor) input pad and the skin of the finger. Since the ridges of the skin are closer to the TFT than the valleys, differing capacitance readings are produced by ridges and valleys. The use of a solid state array as the addressing and readout device for a capacitance imaging system should allow for improved reliability and speed in acquiring a high quality image.
With the invention of CCD (Charge Coupled Device) camera elements, and subsequently CMOS (Complimentary Metal-Oxide Semiconductor) imagers, solid state arrays have taken great strides. Recent developments now include x-ray TFT imagers, where TFT FET (Field Effect Transistor) elements are fabricated as an array on an insulating substrate, and then read out via computer controlled means, and used to create an image. These x-ray images rely on a photodiode to generate a charge when exposed to photonic energy, and that charge can be read out via the array to yield high resolution x-ray images.
For a capacitance imaging system, a means of generating or accumulating a charge in response to a varying distance is needed. These needs are met by the use of a dielectric material configured as a simple capacitor whose voltage is sensitive to the varying distances from the sensing surface to the skin, i.e., the ridges and valleys of the friction ridge surface. This varying charge or voltage may be provided to a peak detecting circuit at the input of an amplifier and row/column readout device, i.e., TFT or CMOS circuit.